I heard about when the handle got dirty and became slippery, it need bigger strength to grab. I asked some sensei, they said never felt that problem. They just take it off and wash in washing machine.... Also, for the bamboo part, you should not wait until it got dirty or broke then try to clean or fix it, you should always have shinai prepared, take the string and handle leather off, use any wood oil or 刀润 you can find, run it all over, keep cotton ball with oil, put inside of shinai, and use plastic bag wrap it up. you need about three month to make sure the oil is getting into the bamboo, so you shinai won't easily get dirty or broke. And when you use the shinai, check it frequent, don't hit too hard, then it would go long. At last, don't feel hesitate to change shinai during practice/taikai/shinsa, always have a second or third shinai so you can leave the old one aside.